Problem: The scale on a map is 3cm : 6km. If the distance between two cities is 36km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 3cm on the map corresponds to an actual distance of 6km. An actual distance of 36km is the same as 6 $\cdot$ 6km. The distance between the two cities on the map is 6 $\cdot$ 3cm, or 18cm.